In the long term, if we eventually make it to the scenario where we stop putting CO2 into the atmosphere on net, then each kg of CO2 input into the atmosphere must be compensated by a kg of CO2 removed by some mechanism or another. The reasonable thing to do in that equilibrium would be to set the price to output CO2 - collected by governments as a carbon fee - equal to the price to capture CO2, paid out by governments as a negative carbon reward.
Of course we could just do that now, the trick is how to get to that equilibrium scenario without collapsing the economy in the process.
I would assume the primary customers would be purchasers of carbon credits, and possibly governments. However, there is likely a large future market for synthetic fuels, and carbon dioxide is an important building block for synthetic hydrocarbons.
The economics is once the wider public realizes how severe climate change really is governments will be willing to pay a very high price to limit the impact.
Yes you're right. One could argue though that those usages would avoid dirty sources of CO2.
But I agree that "avoiding to output more CO2" is not a replacement for "removing CO2", and this is very important indeed. It's just an additional war front, not a replacement.
For an index of 100+ companies working on mining carbon dioxide from the air, check out AirMiners http://www.airminers.org